1. Field of the Invention
This invention relates to video display systems and more particularly to a display refresh memory address generation apparatus for scrolling rows of character information stored in the display system refresh memory in a manner which does not require the reconstruction of the data information as originally stored in the refresh memory.
2. Description of the Prior Art
Information is normally displayed on the cathode ray tube of a display monitor by selectively energizing an electron beam as it scans the sensitized screen of the CRT. The electron beam normally scans the screen from left to right in a succession of horizontal scan paths which begin at the top of the screen and end at the bottom of the screen. The beam is subsequently returned to the top of the screen for the next successive raster scan of the entire screen. This is accomplished by monitor electronics, or beam drive circuitry, associated with the cathode ray tube which magnetically deflects the beam in both the horizontal and vertical directions and selectively energizes the beam as it scans the screen of the CRT. The horizontal retrace of the beam is initiated by a horizontal synchronization (SYNC) signal, the vertical return of the beam to the top of the screen is initiated by a vertical sync signal and the beam is selectively energized in response to a video signal. These signals, the horizontal sync, vertical sync, and video signals are generated by the display controller and transferred to the monitor electronics which in turn uses them to generate the signals which drive the electron beam gun and beam deflection magnets.
In addition to generating the video and sync signals, some displays allow the information to be displayed in a variety of intensities on the CRT screen, for example, a display may allow information to be displayed in normal brightness or in a low intensity mode which is less than the normal brightness. In this case, a low intensity signal must also be generated by the display controller to control the intensity of the information on the display screen. In addition to an intensity mode which may be associated with an individual character or a field of characters which is to be displayed on the display screen, other visual attributes are often found in display systems. For example, an inverse video attribute can indicate that the character of information is to be displayed as a dark character on a light background as opposed to the normal case of a light character on a dark background. A blink video attribute allows the character of information to be blinked on the display screen to draw the display operator's attention to the information. An underline visual attribute allows the character of information in the row to be displayed with an underline under the character. A hide visual attribute results in the blocking of the video signal such that sensitive data will not be displayed on the display screen, although it is available in the refresh memory and may be transmitted or received from a computer attached to the display controller or remotely over a communication line attached to the display controller. In addition, the cursor may be treated as a visual attribute to modify the character which would otherwise be displayed on the display screen to indicate to the operator where the next character of data which is entered from a keyboard attached to the display controller will be placed on the display screen.
The display controller generates the horizontal sync and the vertical sync signals by use of raster scan logic. The video signals are generated by the display controller scanning a refresh memory in the display controller which contains the information which is to be displayed on the CRT screen. The video signals are generated by the display controller scanning the refresh memory a character at a time as each row of information is displayed on the CRT screen. The information within the display controller refresh memory may originate from a keyboard attached to the display terminal, from a computer attached to the display controller, or remotely from a communications line attached to the display controller.
Display controllers have generally stored rows of character information in display refresh memories in a predetermined order. Each row of character information has been read from the refresh memory sequentially in the order stored. In order to scroll the rows of character information on the display screen, a reconstruction of the character information within the refresh memory had been required. More recently, parallel length rows of character information have been stored in the display refresh memory wherein each row contains a trailing firmware code which upon detection by the display controller is used to address the next row of character information which may be randomly located within the refresh memory. Rows of character information may be added, deleted, or re-ordered in any manner without requiring the reconstruction of the character information as stored in the refresh memory. Such a system is described in the U.S. patent application Ser. No. 034,832 entitled "Hardware/Firmware CRT Display Link System" by Joseph L. Ryan and Gerald N. Winfrey, filed on Apr. 30, 1979.
In the present invention, rows of character information are stored in fixed length rows in the display controller refresh memory. The display controller includes a roll register which is loaded with the value corresponding to the number of rows that the information on the display screen is to be rolled (scrolled) and this value is used in generating a relocated address which is used to retrieve the character information stored in the refresh memory. Thus, rows of character information may be scrolled up or down on the display screen without requiring the reconstruction of the character information in the refresh memory other than to blank out the character information in each vacated row.